To ensure optimal output from an excimer gas laser it is necessary to maintain a fixed laser gas or gas mixture composition. Variation of the laser gas or gas mixture composition can occur due to the impurities set up either by the laser operation or by pick-up from components used in the laser, such as plastics. It has been proposed to use a cryogenic trap to remove some impurities by liquifaction, but this technique has the drawback that it does not allow the removal of all possible impurities.
An excimer laser gas mixture typically comprises one or more rare gases such as Helium, Neon, Xenon and Krypton and a halogen containing molecule such as Fluorine or Hydrogen Chloride. A further technique has been proposed for impurity removal by subjecting the laser gas or gas mixture to a high temperature reaction to remove impurities and the halogen. Whilst both the foregoing proposed techniques are successful to some extent in removing impurities from a laser gas or gas mixture, they are not suitable individually for maintaining a control of the composition of a laser gas or gas mixture to within specified limits.